Why does my screen flash then reboot?

I was playing clash of clans when all of the sudden my screen started flashing. It was repetitive and it was black. Then it rebooted all by itself. (I was rooted). I thought immediately to do a factory reset so I did. After doing that. Setting up the device was a pain in the a**. It took like ten minutes because it kept booting my off. The flashing was still there.